Frequency modulated recording and reproducing system



June 10, 1947. c. M. SINNETT FREQUENCY MODULATED RECORDING AND REPRODUCING SYSTEM Filed May l5, 1943 Patented June 10, 1947 FREQUENCY MODULATED RECORDING AND REPRODUCIN G 'SYSTEM Chester M. Snnett, Westmont, .N. J., assignor to Radio Corporation of America, a corporation of Delaware Application May 15, 1943, Serial No. 487,121

8 Claims. 1

My present invention relates to sound recording systems, and more particularly to a home recording system utilizing frequency modulation.

Homev recording and/or reproducing of sound has been accomplished in many ways in the past. The ideal home `recorder should bel capable of producing a relatively inexpensive recording adapted to have a low noise level during reproduction. Frequency modulation has been utilized to translate sounds into mechanical recordings. The recorded impression corresponds in that case toa frequency modulation signal. One of the advantages of such a method of recording-is the low noise level Ydeveloped vduring record reproduction.

It caribe :stated .thatthe main object of my present invention is to provide a home recording system employing a dielectric tape which has directly recorded thereon a frequency modulated signal.

Another important object of this-invention is to provide a novel lmethod of recording sound waves; the method including the steps of embossing on a dielectric record frequency modulation signals derived from vthe waves, frequency modulating a substantially higher frequency oscillation in accordancewith the embossed recording, and subjecting the frequency modulated oscillation to successive discrimination to derive the original sound Waves.

Another object of the invention is to provide a home-recording device which useslow cost material, .such as paper tape, for the recording mediurn, there being recorded'directly-on the paper `tape impression-s representative of frequency modulation signals, reproduction of the tape being provided by translating capacity changes, engendered by the tape impressions, into frequency modulation signals of a high mean frequency, and deriving from the last-mentioned signals the original sounds which were recorded on the tape.

Yet another object of my invention is to provide a relatively noise-free system of reproducing a sound record of the type wherein record impressions exist in a record medium of dielectric material; the system comprising an voscillator whose frequency is varied by virtue of variations in ycaluicity of a condenser Whose dielectric element consists of said record medium, the resulting frequency-variable oscillations being detected in any well-known manner to provide sound currents corresponding to the aforesaid impressions.

A more specic object of my invention to improve reproduction of frequency modulation signalsrecorded on a medium made of dielectric material; the recorded medium being employed as the dielectric element of a condenser composed of a metallic stylus and a'metallic roller, there being provided in series with the condensera second condenser employing said roller as an element thereof; the lirst condenser having a capacitance whichis a relatively small fraction-of the second condenser capacitance; and the series condensers being arranged across an oscillator operating at a relatively high radio frequency thereby to provid-e frequency modulated oscillations whose frequency deviations-areessentially produced by variations inthe capacitance of the first condenser due to the impressions of said record medium. f Y

Still further objects of my inventionare to infi-A prove generally the operation and iidelity of reproduction of home recording systems, and more especially to provide such a system in an economical, eicient and durable manner.

The novel features which I believe to be'characteristic ofmy invention are set for-tn with par-V ticularty in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference -to the following description, toltenin connection Withthe drawing, in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing:

Fig. 1 shows schematically a recording system in accordance with my invention,

Fig. 2 is a magnified view of the hill and dale impression on the record tape, y

Fig. 3 schematically represents the record reproduction system.

Referring now to the accompanying drawing, wherein like reference Icharacters designate similar elements in the different figures, I have schematically represented in Fig. 1 a system for recording sounds in an'economical manner. For example, in home recording, oillce recording and similar uses, it is often desirable to provide a readily-replaceable recording medium. In thje past various forms of simple recording media have been proposed. The methods of recording have been diverse; certain of the methods have proposed the use of frequency modulated (FM) signals as the recorded waves.r In this connection reference is made to my application Serial No. 463,349, led October 26, 1942, for the dis-y closure of an improvedsystem for translating sound waves into FM signals adapted to be recorded on any desired medium.

made of paper or Celluloid. Hence, it will be suiiicient to represent the recorder or embosser in schematic manner.

The numeral I in Fig. l denotes a microphone of any desired form and construction adapted to translate sound waves, whether music or speech, into corresponding audio frequencyV currents. An oscillator 2, preferably operating at a frequency of 15 kilocycles (kc.) produces-oscillations of constant amplitude.

The mean frequency (Fc) may be chosen from a range of the order of 10 to 50 kc. The oscillator frequency is deviated in accordance with the audio currents. That is, the oscillator 2 is subjected to frequency modulation. A so-called reactance tube 3 may be used to vary the oscillator frequency in accordance with the audio currents produced at'the microphone output terminals. Y`

' The reactance tube circuit may be of anywellknown form. Since those skilled in the art of FM are fully cognizant of the circuit details, it need only be explained that the reactance tube simulates or provides a reactive eect. The latter is used to vary thev oscillator frequency, since the tube gain is varied in response to audio current amplitude. The extent of oscillator frequency deviation is a function of audio current amplitude, while the rate of deviation depends on the audio frequenciesY per se. A maximum deviation up to v3 kc. on each side of Fc may be utilized. The invention is not restricted, however, tol such deviation frequencies, nor is it restricted to the specific mean frequency range of 10 to 50 kc. referred to above. I n l The FM oscillatory output of oscillator 2 is passed through an amplitude limiter 4 whose input-output characteristic is depicted above the limiter representation. Any desired form of limiter may be used to eliminate amplitude modulation effects that may exist on the output energy of oscillator 2. The band pass lter 5 provides rejection of carrier frequencyswings exceeding 5 kc. from the illustrative mean or center frequency of 15 kc. The output energy at the output terminals of filter 5 is passed through an amplifier 6. Therefore,l it will be seen that FM signals are supplied tothe recorder device 1. The recorder may be of any well-known form, and is provided with a stylus 8 actuated by an energizing coil V8' fed with the FM signal output of amplifier 6. The stylus 8 functions as an embossing element to emboss on the tape 9 impressions corresponding to the signals.

Y Y The oscillator mean frequency.. may, if desired, be higher or lower than 15 kc.

Numerals IB and I2 denote rollers adapted to guide the tape 9 over theY roller II. The impressions embossed, or engraved, on the upper face of tape 9 are of the hill and dale variety. That is to say, the stylus 8 is subjected to vertical reciprocation in response to the FM signal currents flowing through the energizing coil 8' of the recorder device. In Fig.V 2 I have shownthe contour of the impressions produced by embossing stylus 8 as the tape 9 rides over the roller I I. The impressions 9 are to be understood as being an exact physical representation of the FM signals. The impressions 9 will appear as Shown,

Y gI'OOVe.

in Fig. 2, in cross section, at the center of the Since the tape 9 is paper, which may be properly treated to provide satisfactory embossing, there is provided a cheap and simple form of record medium. It is to be clearly understood that inplace of conventional FM systems there'` may be employed between audio source I and limiter l the ,FM system of my aforesaid application.

`The reproduction of the embossed tape 9 is carried out, according'to my invention, in a man- `ner such that a minimum noise level is produced.

To secure a noise-free reproduction the impressions 9 are translated into capacity variations. 'Ihe latter are, in turn, translated into frequency variations of a high frequency'oscillator. The resulting FM signal energy is then subjected to successive V,demodulations to producel the original sounds which were recorded. These steps are carried out by the system now to be described.

The tape 9 is guided over the face of a metallic roller I3 by the rollers I5 and I l. A metallic stylus I5 is pivotally mounted inthe usual manner so as to permitl the tip of the stylus toiride over the hill and dale impressions of the tape. Since the tape 9 is'made of dielectric material there is provided a condenser consisting of ,theVV stylus I5, dielectric tape 9 and Vmetalroller I3. Y

The capacitance of this condenser is of variable value, and depends upon the variableV thickness of tape 9. Hence the'impressions 9 are translated into capacity changes. The variable condenser is, then, of the type wherein the dielec-r Y tric is the varying element. Y, A second condenser' is arranged in series with the rst condenser. The second condenser includes 'roller I3 as an electrode. The grounded convex metal plate I4 acts as the low potential electrode of the second condenser. Air is the dielectric between metal roller I3 and convex plate I4. f

Merely by way of illustration it is pointed out that the 'capacitance (maximum) of condenser I5-9-I3 may be of the order of 3 to 4 micromicro-farads (mmf). The second condenser IS-M is given a much larger capacitance. For

example,the condenser I3-I4 may be of theV order of mmf. Y These series condensers are` shunted across'the tank circuit I8 of an ultrahigh frequency oscillator I9. The latter mal7 have a mean frequency of 30 megacycles (ma). The specific construction of oscillatorv I9 is not material to my invention, nor is the particular oscillator frequency limiting in any respect; It

is desirable to have the oscillator tank circuit I8v tuned to a normal frequency whichis.. several times higher than the mean'frequency of the'- recordedFMsignals. Y Y Y The frequency'of tank circuit I8 will now be deviated, or modulated, in accordance-with the capacity variations across the series condensers. Since the small variable condenser and large fixed condenser are in series, `the effective capacitance across tank circuit I8 will be equal to the sum of the reciprocals of the separate condensers.

Changes in the capacitance of the small variable condenser will, therefore, bev a substantialportion of the total capacity eiect across the'tank circuit I8. The large xed condenser I3--I4 prevents the occurrence of electro-static noises whichY would arise from the use of a mechanical rubbingv contact. By the use of capacity coupling 'as' shown, a direct mechanical connection is not nec-l eSSaIY.

5. maande .minimum of 29.12I mc. This follows from the fact `that ,the signals 'recorded onl the tape rrepresentf a. :15 kc. oscillatory Wave which isfrequency deviated a, maximum ,andV a. minimumiofg kc..y It will, therefore, be seen. that, the oscillatory; output of oscillator F-S will actuall'yf .be va. Vmodulated wave. In other` words, the ultra-high frequency oscillations whose mean frequency is. 30- mn.. will 4be frequency modulated by a primary frequency modulation signal. This double modulation method of reproducing also tends to. maintain a low noise level. The reason for double frequency modulation can best be explained by stating that if audio frequencies of theA order of 15 kc. were used, the small capacity change caused by the stylus riding in the groove wouldr not'be sufficient to produce a frequency shift .which could be later demodulated. By using a radio frequency a larger degree of frequency shift is obtained directlyfrom the capacity-variations caused by tape; thickness, and in this way the superaudible frequency modulation signals recorded on the tape-have sufficient amplitude to be again demodulated.

The. frequency modulated 'out-put of oscillator lj!! 'is subiected vto the `action of any well-known form of discriminator-rectiiier 20 in order to derive therefrom the primary FM signals which Y were recorded on the tape 9. Above the rectangle 20 there is shown the typical and ideal S-shaped characteristic which is representative of the action of an FM detector. For example, there may be used for this detector the type of FM detector shown by S. W. Seeley in his U. S. Patent No. 2,121,103, granted June 21, 1938. The output of the network 20 is passed through a band pass filter 2l so as to eliminate any frequency deviations which are less than 10 kilocycles and more than 20 kilocycles. It will be recognized that the function here is similar to that described in connection with band pass filter 5 in Fig. 1. The filter 2l keeps out noise modulations below kc. A high pass filter above 10 kc. would probably prove to be as satis factory. If surface noise and other noises below 10 kc. are allowed to come through the system, then they will be reproduced in the audio output.

In order to eliminate any amplitude modulation effects which may have arisen, there is employed an amplitude modulation limiter 22. A second discriminator-rectier 23 is then employed toY derive the original modulation from the low frequency carrier. This network 23 may be constructed in the same manner as network 20. However, whereas network 20 may have a mean frequency of 30 mc., the network 23 will have a mean frequency of kc. The modulation output of network 23 will be the audio frequency currents which existed at the output terminals of the microphone l in Fig. 1 at the time a particular record was made. Any desired type of audio frequency amplifier 24 may now be employed to amplify the audio currents. The amplied audio currents may then be reproduced by any well-known form of loud speaker or other sound reproducer.

It will be clearly understood that I have omitted from the schematic showings of Figs. 1 and 3 the specific driving mechanisms for actuating the tape `9. These are very vWell known, and are not deemed to be essential to a proper description of this invention. Furthermore, it will be clear that the tape 9 when provided in the form of rolls will be capable of considerable usage, and can be manufactured sufficiently cheaply so as to warrant vdestruction of thetape when it has served its purpose. Further, the signals maybe recorded' directly on the film, or tape', at` audio frequencies.

While I have indicated and described several systemsfor carrying my invention into effect,` it will,a be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made with` outy departing from the scope of myA invention.

What I. claim is:

l. In acapacity pickup device: a metallic stylus, ra metallic supportya record consisting` oiga.. relatively thin tape composed of paper a face of said. tape being provided wit-ha plurality of embossed impressionsv in theform of hills and dales, said tape being positioned to move be.` tween said support and stylus with the stylus riding in said impressions, and said impressionsbeing representative of frequency modulation signals. j

2. A method of recording soundy waves-which includes: the steps ofv embossingeon a dielectric recording` medium frequency modulation signals derived from the sound waves, frequency modulating a substantially higher frequency oscillation in accordance with the embossed recording, and subjecting the frequency modulated oscillation to successive demodulations to derive the original sound waves.

3. In combination with an oscillator of relatively high mean frequency, means for modulating the oscillator comprising a pair of condensers arranged in series across the oscillator tank circuit, one of said condensers having a substantially smaller capacitance than the other condenser, a record made of dielectric material functioning as the dielectric of said s mall condenser, and one of the electrodes of the small condenser functioning as a stylus for impressions on the dielectric material.

4. A method of reproducing a record whose impressions corresponds to frequency modulation signals of a low mean frequency, translating the impressions into corresponding capacity variations, generating oscillations of a relatively high mean frequency, translating the ycapacity variations into deviations of said relatively high frequency oscillations, dernodulating the resulting frequency-deviated oscillations to provide the recorded frequency modulated signals, and subjecting the frequency modulation signals to further demodulation to derive therefrom sounds corresponding to the modulation on the original frequency modulation signals.

5. In combination with an oscillator of relatively high mean frequency, means for modulating the oscillator comprising a pair of condensers arranged in series across the oscillator tank circuit, one of said condensers having a substantially smaller capacitance than the oth'er condenser, a record made of dielectric material functioning as the dielectric of said small condenser, one of the electrodes of the small condenser functioning as a stylus for impressions on the dielectric material, and a metal guide roller acting as the common electrode for both condensers.

6. A method of reproducing a record whose impressions correspond to frequency modulation signals of a low mean frequency, translating the impressions into corresponding capacity variations, generating oscillations of a relatively high radio frequency, translating the capacity variations into deviations of said relatively high' radio frequencyoscillations, demodulating the resulting frequency-'de'viated oscillations to vprovide Vthe recorded frequency modulated signals, filtering the latter through a band Ypass network, subjecting the frequency modulation signals to further Y demodulation to derive therefrom sounds corresponding'to the modulation on the original frequncy modulation signals.

7; In combination with an oscillator of rela-f tiyely'high radio frequency of the orderA of 30 megacycles, means for modulating th'e oscillator comprising a pair of capacities arranged in series across the oscillator tank circuit, one of said capacities having a substantially smaller capacitance than the other capacity, a record 8. A method of reproducing a record whose impressions correspond to frequency modulation signals of a low radio frequency, translating the4 impressions into corresponding capacity variations, generating oscillations of a relatively high radio frequency, translating vthe 'capacity varia tions into deviations of said relatively high radio frequency oscillations, denodulating the resulti-ng'frequency-deviated oscillations to Yprovide the' recorded frequency modulatedsignals, an'd'deriv-Y Y ing from the latter sounds corresponding-to the'` modulation on the original frequencyV-modulation'signals. Y 'n Y Y Y CI-IES'I'ER, M. SINNEIT.V

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